The FBI has uncovered new emails related to Hillary Clinton's use of a private email server, prompting federal authorities to investigate them.

The FBI discovered the emails as part of an "unrelated case," FBI Director James Comey said in a letter to a congressional committee that was later tweeted on Friday.

These emails "appear to be pertinent" to the FBI's original investigation into Clinton's private server use, which the agency wrapped up back in July, Comey said. Clinton, now the Democratic nominee for U.S. president, used the privacy server while she served as secretary of state.

"She was entrusted with some of our nation's most important secrets, and betrayed that trust by carelessly mishandling highly classified information," he said in a statement.

He's asking the U.S. director of national intelligence to suspend all classified briefings with Clinton until the matter is resolved.

Clinton and her presidential campaign have yet to respond to the FBI's new investigation.

In July, the FBI concluded that Clinton had been "extremely careless" in her use of a private email server, but the agency didn't recommend filing any charges against her.

The FBI said Clinton's server faced ongoing cyber threats from possible hackers, including phishing email attacks and failed login attempts. However, the agency found no evidence confirming that the server was ever compromised.

The letter from FBI's director didn't mention how the newly uncovered emails were obtained or where they came from.

However, recently stolen emails from a Clinton aide have been published through WikiLeaks and include allegedly thousands of private messages between U.S. officials and her staff.